1. Field of the Invention
The present invention relates to a color matching method, and in particular, to a perceptual color matching method between two different polychromatic displays.
2. Description of the Related Art
Current display technology employs the characteristics of human vision to regenerate the color, instead of emitting light beams at the exact frequency corresponding to each color. Therefore, a display with different configurations of color primaries, it requires a corresponding color mapping algorithm. The simplest way to generate a color is to use three-color primaries, such as red (R), green (G) and blue (B) on light emitting displays, as shown in FIG. 1.
FIGS. 1 and 2 show a conventional R′G′B′-stripe display and a RGBW pattern display. Recently, there is a lot of research working on color matching from RGB to RGBW screen. The RGBW serene comprises four color primaries, such as red (R), green (G), blue (B) and white (W). However, color images of the RGBW display may be different from those of conventional R′G′B′-stripe display due to different color gamuts. Color matching is essential in order to match the performance.
Referring to FIGS. 1 and 2, we assume the size of both full-pixels to be the same. Even if they use the same power of backlight, both color gamuts will intersect each other, but not exactly overlapped. Thus, a color cannot be converted exactly from one to another unless increasing the range of luminous intensity such that the target color gamut to be a superset of source color gamut. Most of the colors matching algorithms exploit clipping or re-mapping of colors.
In the conventional color matching method, suppose (R′, G′, B′) be the values of the color in the conventional R′G′B′-stripe display and (R, G, B, W) be the corresponding values in RGBW display. The simple way to match it as the equation shown below:
      R    =                  4        3            ⁢              R        ′              ,      G    =                  4        3            ⁢              G        ′              ,      B    =                  4        3            ⁢              B        ′              ,      W    =    0  
The 4/3 factor is used because it is a ratio of area of sub-pixel between two panels. However, if the value is greater than three quarter of the full strength, the value of the new display will become overflow. Therefore, we must clip the value to make sure the new set of values within the valid range. On the other hand, there is an extra white color dot, which can be decomposed into r, g, b.W=krr+kgg+kbb, kr+kg+kb=1
Where kr, kg, kb are respectively the coefficients of red, green and blue components of luminance of a color space. For example, (0.299, 0.587, 0.114) is used in NTSC standard. As some portions of R′, G′, B′ will combine to become luminance, the conventional color matching method shifts the common amount in R′, G′, B′, into the color dot W. That is similar to the approach proposed by Morgan et. al.
  W  =                                                        4              3                        ⁢            min            ⁢                                                  ⁢                          (                                                R                  ′                                ,                                  G                  ′                                ,                                  B                  ′                                            )                                ,                      R            =                                                            4                  3                                ⁢                                  R                  ′                                            -              W                                ,                                                          G            =                                                            4                  3                                ⁢                                  G                  ′                                            -              W                                ,                      B            =                                                            4                  3                                ⁢                                  B                  ′                                            -              W                                          
Allocating color into the extra white dot can also free up more vacant spaces in R, G, B color primaries for further color enhancement processes. However, if the difference between R′, G′, B′ is large, some of values of R, G, B may still be overflow. Therefore, it requires an additional process to transform the values of colors outside the gamut falling into the range of the target gamut. Morgan et. al. suggested to clip the colors. Then many overflowed values are mapped to a single value. The color matching function becomes non-injective. It results that no distinction is found in those overflow values. To avoid such a deficiency, Tanioka proposed a method of reduction of a scale in a portion of values so that the overflow values can be compressed within the valid range. This contraction method suffers from the reduction of the changes of color and hence reduces the contrast. On the other hand, Lee et. al. proposed a re-mapping method with the preservation of hue and saturation. Although this method can keep the chrominance, it sacrifices the importance of luminous intensity. As mentioned above, it is impossible to exactly match all the values of two non-overlapping color gamuts.
U.S. Pat. Nos. 6,885,380 and 6,897,876 disclose a method for transforming three color input signals to four or more color output signals. According to the spatial arrangement of sub-pixels within a full-pixel, a color coordinate conversion matrix is introduced in order to convert all the colors into XYZ color space. Though the computation is simpler and it can retain the accuracy of converting colors within the overlapping part of color gamuts, the color outside gamut still has a problem of diminishing the luminance and degradation of color accuracy. The method used in U.S. Pat. Nos. 6,885,380 and 6,897,876 considers only the configuration of a single full-pixel but neglect the color interference caused by surrounding pixels.
Therefore, it is necessary to provide a color matching method to solve the above-mentioned problems.